Can now retrieve fields from event

Linked to #103 

My configuration file look like this : 
```
input {
  gelf {
    port => 12201
    type => gelf
  }
}

output {
  influxdb {
    host => "X.X.X.X"
    port => 8086
    db => "metrics"
    user => "user"
    password => "password"
    series => "%{message}"
    use_data_points => false
    ignore_data_points => ['@timestamp']
    data_points => {}
  }
}
```

Th influxdb output can now automatically use all fields available in event object and send it to influxdb.